1. Field of the Invention
This invention relates to a semiconductor device having an active circuit constituted by thin film transistors formed over a substrate, and a method of fabricating the semiconductor device. More particularly, this invention can be used appropriately for fabricating thin film transistors using a crystalline semiconductor layer. The present invention can be utilized also for fabricating an integrated circuit that comprises thin film transistors, an electro-optical device typified by an active matrix display device, and an image sensor, and an electronic appliance having the electro-optical device mounted thereto.
2. Description of the Related Art
A semiconductor device typified by an active matrix liquid crystal display device comprising a large number of thin film transistors (TFTs) arranged on a substrate has been developed. In order to accomplish high mobility in the TFTs, it has been believed preferable to use a crystalline semiconductor film for a semiconductor layer. Most of polycrystalline semiconductors that are utilized for the TFT are formed by crystallization technologies such as laser annealing and thermal annealing. However, because a large number of defects exist in the crystalline semiconductor film so crystallized, mobility of the carriers and the life time are markedly lowered with the result that electric characteristics of the TFT are adversely affected.
To eliminate the defects inside the crystalline semiconductor film and an inter-layer insulating film, a hydrogenation process has been known as one of the effective means. The hydrogenation process includes a plasma hydrogenation process that neutralizes the defects by generating hydrogen plasma, and a hydrogenation method that executes heat-treatment in a hydrogen atmosphere. These hydrogen process steps are appropriately incorporated in the fabrication process steps of the TFT.
According to the hydrogenation process utilizing the plasma, however, the crystalline semiconductor film is directly exposed to the plasma in order to effectively introduce hydrogen. In consequence, there remains the problem that the crystalline semiconductor film is damaged by the ions that are formed simultaneously in the plasma. To recover this damage, heat-treatment at 400 to 600xc2x0 C. is believed necessary, but when heating is made to 400xc2x0 C. or above, hydrogen is re-emitted from the crystalline semiconductor film. If any atmospheric components such as nitrogen and oxygen remain in the atmosphere in the plasma hydrogenation method, these elements, too, are converted to the plasma and contaminate the crystalline semiconductor film.
When the plasma hydrogenation method is carried out from the surface side of an inter-layer insulating film formed on a gate electrode, the defects existing inside the crystalline semiconductor film can be neutralized to a certain extent. However, the hydrogen concentration introduced into the film by this method has a distribution such that it progressively decreases from the surface to its inside. For this reason, it has been difficult to sufficiently hydrogenate the crystalline semiconductor film on the lower layer side. The heat-treatment process in the hydrogen atmosphere as another method involves the problem that the process time gets unavoidably extended in order to improve the hydrogenation effect.
The present invention is directed to provide a high performance semiconductor device that can solve the problems described above, and can be efficiently processed by a hydrogenation process without imparting damage and contamination of a crystalline semiconductor film, and a method of fabricating such a semiconductor device.
A method of fabricating a semiconductor device according to the present invention comprises the steps of forming a hydrogen-containing first insulating film over a semiconductor layer that is formed into a predetermined shape over a substrate, and then conducting heat-treatment in a hydrogen atmosphere or in an atmosphere containing hydrogen that is formed by generating hydrogen plasma. The first insulating film may be an inter-layer insulating film formed over a gate electrode. When hydrogenation is carried out through this first insulating film, the problems of damage to a semiconductor layer and its contamination can be avoided. Because hydrogen is supplied to the hydrogen-containing first insulating film, hydrogen in this first insulating film diffuses into its lower layer and hydrogenation of the semiconductor layer proceeds.
Another method of fabricating a semiconductor device according to the present invention comprises the steps of forming a hydrogen-containing first insulating film over a semiconductor layer that is formed into a predetermined shape over a substrate, forming a second insulating film in close contact with the first insulating film, and then executing heat-treatment in a hydrogen atmosphere or in an atmosphere containing hydrogen that is formed by plasma generation. Hydrogen that is supplied from the surface of the second insulating film diffuses into the first insulating film, and hydrogen in the first insulating film diffuses into the lower layer, so that the semiconductor layer can be hydrogenated. In this case, the heat-treatment may be carried out in the hydrogen atmosphere or in the atmosphere containing hydrogen formed by plasma generation, after the first insulating film is formed.
Another method of fabricating a semiconductor device according to the present invention comprises the steps of forming a hydrogen-containing first insulating film over a semiconductor layer that is formed into a predetermined shape over a substrate, conducting then heat-treatment in a hydrogen atmosphere or in an atmosphere containing hydrogen formed by plasma generation, forming a hydrogen-containing third insulating film on the first insulating film, and conducting heat-treatment in an atmosphere containing hydrogen or nitrogen. When the third insulating film comprises a compact film of a silicon nitride oxide or a silicon nitride film, the quantity of hydrogen dissociating from the first insulating film into the gaseous phase can be reduced and hydrogenation of the semiconductor layer can be attained more reliably.
More preferably, the method of fabricating a semiconductor device comprises the steps of forming a hydrogen-containing first insulating film over a semiconductor layer formed into a predetermined shape over a substrate, conducting then heat-treatment in a hydrogen atmosphere or in an atmosphere containing hydrogen that is formed by plasma generation, forming a second insulating film in contact with the first insulating film, conducting heat-treatment in a hydrogen atmosphere or in an atmosphere containing hydrogen formed by plasma generation, and conducting heat-treatment in an atmosphere containing hydrogen or nitrogen after the hydrogen-containing third insulating film is formed on the second insulating film. According to this construction, hydrogen that is supplied by the step of conducting the heat-treatment in the hydrogen atmosphere or in the atmosphere containing hydrogen formed by plasma generation diffuses into the lower layer, and hydrogenation of the semiconductor layer can be effected reliably.
The first insulating film is preferably a silicon nitride oxide film made from silane, nitrous oxide or ammonia. The second insulating film may be a silicon nitride oxide film made from silane or nitrous oxide. The third insulating film is preferably a silicon nitride oxide film made from silane, nitrous oxide or ammonia, or a silicon nitride film made from silane, ammonia or nitrogen. All the first to third insulating films fabricated in this way have the carbon concentration in the films of not greater than 2xc3x971019cmxe2x88x923.
Therefore, the semiconductor device according to the present invention comprises, over a semiconductor layer formed into a predetermined shape, a first insulating film comprising a silicon nitride oxide film having a hydrogen concentration of at least 1 atomic % to less than 30 atomic % and a nitrogen concentration of at least 10 atomic % to less than 25 atomic %, and a third insulating film keeping contact with the first insulating film and comprising a silicon nitride oxide film having a hydrogen concentration of at least 1 atomic % to less than 30 atomic % and a nitrogen concentration of at least 10 atomic % to less than 25 atomic % or a silicon nitride film having a hydrogen concentration of at least 1 atomic % to less than 30 atomic %.
The semiconductor device may comprise, over a semiconductor layer formed into a predetermined shape, a first insulating film comprising a silicon nitride oxide film containing at least 10 atomic % to less than 30 atomic % of hydrogen, and having a nitrogen concentration of at least 10 atomic % to less than 25 atomic %, a second insulating film comprising a silicon nitride oxide film, keeping contact with the first insulating film and having a nitrogen concentration of less than 10 atomic %, and a third insulating film keeping contact with the second insulating film and comprising a silicon nitride oxide film having a nitrogen concentration of at least 1 atomic % to less than 25 atomic % or a silicon nitride film having a hydrogen concentration of at least 1 atomic % to less than 30 atomic %.
In a semiconductor device including a gate insulating film formed in contact with a semiconductor layer shaped into a predetermined shape and a gate electrode formed at a predetermined position on the gate insulating film, a semiconductor device according to the present invention comprises a first insulating film keeping contact with the gate insulating film and with the gate electrode and comprising a silicon nitrogen oxide film containing at least 1 atomic % to less than 30 atomic % of hydrogen and having a nitrogen concentration of at least 10 atomic % to less than 25 atomic %, and a third insulating film keeping contact with the first insulating film and comprising a silicon nitride oxide film having a nitrogen concentration of at least 1 atomic % to less than 30 atomic % and a nitrogen concentration of at least 10 atomic % to less than 25 atomic %, or a silicon nitride film having a hydrogen concentration of at least 1 atomic % to less than 30 atomic %.
In a semiconductor device including a gate insulating film so formed as to keep contact with a semiconductor layer formed into a predetermined shape and a gate electrode formed at a predetermined position on the gate insulating film, a semiconductor device according to the present invention comprises a first insulating film comprising a silicon nitride oxide film so formed as to keep contact with the gate insulating film and with the gate electrode, containing at least 1 atomic % to less than 30 atomic % of hydrogen and having a nitrogen concentration of at least 10 atomic % to less than 25 atomic %, a second insulating film keeping contact with the first insulating film and comprising a silicon nitride oxide film having a nitrogen concentration of less than 10 atomic %, and a third insulating film keeping contact with the second insulating film and comprising a silicon nitride oxide film having a hydrogen concentration of at least 1 atomic % to less than 30 atomic % and a nitrogen concentration of at least 10 atomic % to less than 25 atomic %, or a silicon nitride film having a nitrogen concentration of at least 1 atomic % to less than 30 atomic %.